The present invention relates to a flammable-gas sensor.
Conventionally, for a flammable-gas sensor, there has been proposed a device wherein, as shown in FIG. 13, an element 21, such as SnO.sub.2, which is sensitive to flammable gases and whose resistance changes as it reacts to such gases, is arranged in series with a direct current source 22 and an ammeter 23. A heater 24 which is designed to heat said element 21 up to approximately 100.degree. C.-400.degree. C. is connected in series with an electric source 25. Another proposed device consists of, as shown in FIG. 14, a casing 31 which contains an aqueous solution of sulfuric acid 32 which acts as an electrolyte. The casing 31 has openings 33, 34 and is provided with gas-permeable polytetrafluoroethylene film 35, 36 at set positions inside said casing. On the inner side of said gas-permeable polytetrafluoroethylene film 35, 36 there are formed at set positions a working electrode 37, a counter electrode 38 and a reference electrode 39, all formed by a coating of platinum black. The electric potential between the working electrode 37 and the reference electrode 39 is maintained constantly by means of a potentiostat circuit 40.
Still another device proposed is a hydrogen-gas sensor of the field-effect transistor type (hereinafter abbreviated as "FET type") wherein the gate electrode of the field-effect transistor is formed of palladium (Pd).
However, problems have been involved in the operation of these devices. A flammable-gas sensor shown in FIG. 13 is designed to produce current signals corresponding to the amount of adsorption of flammable-gas particles. However, saturation of the flammable-gas particles that occurs when the adsorption reaches a certain amount renders the sensor unable to continue to detect the concentrations of the flammable gas. Furthermore, the necessity of heating up the element 21 to approximately 100.degree. C.-400.degree. C. by use of heater 24 in order to detect flammable gas not only makes the device complicated in design, it also may a gas explosion. Complication characterizes such a flammable-gas sensor, both in production and adjustment for use, because the element 21, influenced by production conditions, tends to result in wide variations in characteristics, and consequently, it becomes necessary to adjust the compensating circuit to the varied characteristics of the element. The electrical heating which is indispensable before a sensor becomes ready for functioning requires much time and the operation requires much electrical energy. The selectivity for detecting varieties of gases is poor; the sensor reacts similarly to flammable gases of like nature, i.e. gases such as hydrogen gas and a hydrocarbon gas. For example, a sensor, though designed to detect a leak of flammable gas, also reacts to the alcohol gas which rises from the pot when it is warmed and, in such cases, may erroneously detect such gas as a leak of flammable gas.
On the other hand, with respect to a flammable-gas sensor shown in FIG. 14, the characteristics of the gas-permeable polytetrafluoroethylene film, those of the electrodes, the accuracy of the positions of the electrodes, etc. altogether have such influences that each sensor is produced with irregularities in characteristics as a whole. The production, therefore, involves painstaking work to conform the characteristics, the accuracy of the positions, etc. to the set values. Nevertheless, it is practically impossible for each sensor produced to have uniform characteristics.
It is also a problem that the speed of the reaction, influenced by the concentration of the gas that permeates the gas-permeable polytetrafluoroethylene film, can not be substantially increased. Furthermore, with an electrolyte being contained in the casing, there is likely to occur a leakage of the electrolyte after a long period of use. The leakage impairs the reliability of a sensor in its function to detect flammable gases. This defect is conspicuous when the application is that of a portable sensor. It is virtually impracticable, therefore, to generally reduce sensors dimensionally and lower the production cost.
A hydrogen-gas sensor of the FET type is so designed that the catalytic action of palladium (Pd) causes H.sup.+ ions to be formed by decomposition and to accumulate only at the gate electrode. In this method, said accumulation provides a basis for measuring the change of the concentration of hydrogen gas, but not for measuring the value of the absolute concentration.
With the problems pointed out in the above description taken into consideration, the present invention has for its object to provide a flammable-gas sensor which is satisfactory in structural simplification, in minimizing irregularities in characteristics, in improving the selectivity for flammable gases, and in ensuring a high accuracy in measurement over long periods of time.